Chapter 12

Chapter 12

1. Waves and Vibrations

   • Types of Waves
     • Mechanical Waves
       • Sound waves, seismic waves and waves on a string - waves that pass through a substance
     • Electromagnetic Waves
       • vibrating electric and magnetic fields that progress through space without the need for a substance
   • Londitudinal and Transverse Waves
     • Longitudinal Waves
       • The direction of vibration of the particles is parallel to the direction in which the waves travels. Sound waves, primary seismic waves and compression waves on a 'slinky'
     • Transverse Waves
       • The direction of vibration is perpendicular to the direction in which the waves travels. Electromagnetic waves, secondary seismic waves and waves on a string or a wire are all transverse waves
   • Polarisation
     • Transverse waves are plane-polarised if the vibrations stay in one plane only. If the vibrations change from one plane to another, the waves are unpolarised. Longitudinal waves cannot be polarised.
     • Intensity of light

2. Measuring Waves

   • Key Terms
     • Displacement
       • Particle's distance and direction from its equilibrium position 
     • Amplitude
       • maximum displacement of a vibrating particle.
     • Wavelength
       • least distance between two adjacent vibrating particles with the same displacement 
     • One complete cycle of a wave
     • Period
       • time for one complete wave to pass a fixed point
     • Frequency
   • Wave speed
     • c = f x wavelength(lamda)
   • Phase Difference
     • phase difference in radians =  (2 x pi x d)/ wavelength(lamda) Fraction of a cycle between two particles

3. Waves Properties

   • Reflection
     • Straight waves directed at a certain angle to a hard flat surface (the 'reflector' reflect off at the same angle.
   • Refraction
     • When waves pass across a boundary at which the wave speed changes, the wavelength also changes. If waves fronts are at a non-zero angle to the boundary, they change direction as well as changing speed.
   • Diffraction
     • Waves spread out after passing through a gap or round an obstacle. The narrower the gap, the more the waves spread out. The long the wavelength, the more the waves spread out.
     • Dish design
   • Superposition
     • When two waves meet, the total displacement at a point is equal to the sum of the individual displacements at that point.
     • Supercrest/Supertrough
     • Stationary waves on a rope
     • Water waves in a ripple tank

4. Stationary and Progressive Waves

   • Formation
     • Two opposing waves (progressive) combine to form a standing wave. This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling in opposite directions
   • Explanation
     • Amplitude
       • varies from zero at the nodes to a maximum at the antinodes
     • The phase difference
       • Equal to m*(pi) where m is the number of nodes between the two particles